This invention is of significance in the treating of molten metal in the aluminum industry, with particular reference to the transferring of molten pot metal to furnaces or chambers from which the air has been evacuated.
The purpose of the present invention is to provide the equipment required for performing this operation.
The electrolytic reduction of alumina to aluminum takes place in large electrolytic reduction cells, or pots, at a temperature of about 970.degree. C., and with production per 24 hours of around 1 ton. A potroom contains between 100 and 180 such pots. The chemical composition varies somewhat from one pot to another, and for this reason the metal taken from several pots is mixed in order to arrive at the composition required for the finished product.
The metal is tapped from the pots by a tube which is immersed into the metal, the other end of which tube is fixed into the top of a tapping crucible. For tapping, the air is evacuated from this crucible and the metal is sucked up. The metal is then conveyed, in the tapping crucible, to a mixer furnace.
It has been usual practice to use one mixer to serve two casting furnaces. In the casting furnace, the metal can be cleaned in various manners before pouring. The furnace can be put under vacuum in order to allow gas to escape while the metal stands undisturbed for the required period. The vacuum treatment can be made more efficient by allowing the metal to be sucked into the furnace. The metal entering the furnace in the form of a jet exposes a large and ever-changing surface of the metal. The diameter of the jet is small, favouring a rapid liberation of gases. The vacuum treatment can also be enhanced by introducing inert gases down into the metal while the furnace is under vacuum. The vacuum treatment of metal through its being sucked into a vacuum furnace is very efficient, but has hitherto required that the metal be added continuously from a mixer. It has not been possible in practice to seal the charge hole when the furnace is in use and under vacuum.